This invention is concerned with a method and apparatus for conservation of water.
The invention is particularly concerned with the reduction of evaporation losses from water storages having a high ratio of surface area to water depth.
In many regions of Australia and elsewhere in the world, the capacity for sustainable horticulture is dependent on the availability of water.
In arid and semi-arid regions, a level of sustainable horticulture has been achieved by building large but relatively shallow water storage dams covering many hectares.
Water levels in such dams can be topped up in rainy seasons by drainage from catchment areas where the topography is appropriate or otherwise by pumping water from creeks or rivers when water is flowing therein.
A major disadvantage of such water storage systems is the high rate of water loss due to evaporation due to the combined effects of wind and water surface temperature.
Evaporative losses are generally measured in megalitres/hectare where a 100 mm reduction in water depth per hectare equals one megalitre.
In semi-arid areas where average annual rainfall may be of the order of 600 mm, evaporative losses during the summer are typically of the order of 18 megalitre/hectare or a reduction in water depth of 1.8 metres.
In more arid areas where average annual rainfall may be 200 mm or less, evaporative water losses of up to 30 megalitres/hectare have been recorded.
While the proportion of water lost by evaporation in water storage facilities can be reduced by increasing the depth/surface are ratio, this is generally uneconomical.
For large capacity water storage dams of many hectares in surface area, these are usually constructed on flat land (without a surrounding catchment area) by pushing up a perimetral wall of 2-3 metres in height with a bulldozer. It generally is not economically feasible to excavate large volumes of earth to form a water storage facility.
As far as the cost of evaporative losses are concerned, these may be measured by the cost of water purchased and/or the value of lost agricultural production.
Typically, in an irrigation area where water is pumped from a stream, the cost of a water allocation license may cost from $1000-$3000 as an initial fee and a seasonal pumping cost of about $25 per megalitre subject to volumetric limits. These costs are steadily increasing as water becomes scarcer due to seasonal variations and increased levels of horticulture.
If evaporative losses were to be measured in terms of lost agricultural production otherwise possible, the value per megalitre of water could range between $500 for a cotton crop up to $1000 or even higher for high value crops such as vegetables or the like.
Another problem associated with evaporative losses from open storage ponds is the risk of increased salinity in water applied to crops as water levels diminish due to evaporation. This problem can be exacerbated where the water is constantly held in storage i.e. the storage pond is never completely emptied to remove accumulated salt concentrations.
Over the years there has been extensive research into reduction of evaporative water losses.
Prior art proposals have included chemical, physical, and structural methods.
Typically, chemical methods comprise the use of a chemical monolayer on the water surface to reduce the evaporation rate. The most well known of these is the use of cetyl alcohol.
While chemical monolayers have proven useful in pilot studies on small surface areas, there are real practical difficulties in maintaining the integrity of the monolayer due to wind actions well as contamination and biodegration of the monolayer.
Physical methods of evaporation control include destratification to bring cooler water to the surface, however, this is of little value in reducing evaporative losses due to wind action.
Other physical methods have involved floating covers made from:
expanded perlite ore
polystyrene beads
foamed wax blocks
white spheres
butyl rubber sheets painted white
polystyrene sheets and rafts
white foamed wax in continuous layers
foamed butyl rubber
light grey asphaltic concrete blocks.
While encouraging results have been obtained with some of these systems (up to 80% reduction with floating concrete rafts) none are suited to very large water storages having a surface area of many hectares due to cost.
Structural methods including roofing of reservoirs have shown evaporation reductions of up to 90% but again, the cost of such structures is not feasible for large surface areas.
Accordingly, the present invention seeks to overcome or ameliorate at least some of the disadvantages of prior art water evaporation reducing systems and to provide, if not a more cost effective system, at least a useful alternative choice.
According to one aspect of the invention, there is provided, a system for reducing evaporation losses in a large surface area water storage, said system comprising:-a buoyant flexible membrane extending over a substantial portion of the surface of a body of water, said membrane being anchored by flexible anchoring means spaced about the periphery thereof and connected to a peripheral wall of said water storage, said membrane characterized in that it comprises a plurality of membrane elements engageable along respective adjacent edges thereof, said membrane further characterized in the provision of spaced apertures to prevent, in use, accumulation of rain water on an upper surface thereof.
Suitably, the flexible membrane is comprised of a natural or synthetic polymeric material.
If required, the flexible membrane may comprise a closed cell foam structure for buoyancy.
Alternatively the flexible membrane may comprise spaced buoyancy chambers.
Preferably the spaced buoyancy chambers extend over at least one surface of said membrane.
Most preferably the buoyancy chambers extend over a surface of said membrane, in use, in contact with the surface of the body of water.
The buoyancy chambers may be interconnected if required.
The membrane elements suitably comprise parallel sided members having telescopically engageable connection means extending adjacent opposed longitudinal edges.
Suitably the telescopically engageable connection means comprises an elongate socket-like element extending adjacent one edge of said membrane element and an elongate spigot-like element extending adjacent an opposite edge, each said socket-like element and spigot-like element being telescopically engageable in a respective complementary connection means of an adjacent membrane element.
Alternatively the membrane elements may comprise connection members spaced along opposite sides thereof. If required, the connection members may comprise apertured eyelets, interengageable hooks and eyes or hooks and eyes engageable by a cord member.
The flexible anchoring means suitably comprises cord-like members adapted for attachment to spaced anchor members located about the peripheral wall of said water storage.
According to another aspect of the invention there is provided a method of reducing the evaporative losses in a water storage, said method comprising the installation in a large surface area water storage of a system according to the first aspect of the invention.